In recent years, optical communication systems having a wide frequency band together with wavelength division multiplexing or bidirectional transmission have realized high speed and advanced functions in public communication and computer networks.
In the field of optical communication, optical integrated circuits having various functions have been studied in order to perform advanced optical signal processing. An optical integrated circuit is basically formed by an optical waveguide, which propagates light by covering a core region. The core region has a high refractive index with a clad layer having a relatively low refractive index and hereby confining the light in the core region and realizing various functions of patterning and arranging cores. Particularly, a quartz-based optical waveguide has many advantages such as low loss, physical chemical stability, and adjustability with an optical fiber and serves as a typical passive optical waveguide.
A typical optical-waveguide forming method uses the flame deposition method as a core-clad-film forming method and the reactive-ion etching method as a core-pattern forming method. The CVD method, vacuum deposition method, and sputtering method are proposed as core-clad forming methods in addition to the flame deposition method.
Though many methods as proposed as described above, yet have achieved an optical-waveguide forming method having high performance, mass productivity, and low cost. This is because each of the film forming methods has both advantages and disadvantages. For example, a high-quality core can be formed by the flame deposition method or CVD method. However, the flame deposition method requires high-temperature annealing at 1,0000° C. or higher for more than ten hours a plurality of times and the CVD method has a difficult point in mass production that a film-forming area is narrow. Moreover, though the electron-beam deposition method or sputtering method can realize small-loss film formation, there is a problem in cost as an optical-waveguide forming process normally requiring a film thickness of ten to tens of microns because a film forming rate is low.
To solve the above problems on formation of an optical waveguide, it is a known optical-waveguide process to form a groove on a substrate serving as a lower clad. The groove is filled with a material having a higher refractive index than the substrate, and the groove is used as a core because the core can be realized in a short time.
FIGS. 5(a) to 5(c) show the above type of optical-waveguide forming method. First, as shown in FIG. 5(a), a substrate 51 on which an optical-waveguide groove is formed is filled with a high-refractive-index material 52 used as a core.
Then, as shown in FIG. 5(b), extra high-refractive-index material is removed from the high-refractive-index material having been filled in the step shown in FIG. 5(a).
Then, as shown in FIG. 5(c), a clad substrate 53 and the optical-waveguide-groove substrate 51 are finally bonded together.
However, the above optical waveguide has the following problems with respect to cost and performance.
In the case of the forming method of filling an optical-waveguide groove shown in FIG. 5, if a high-refractive-index material is present as an adhesive layer, the light confined in a core leaks to the adhesive-layer portion. Therefore, the step of removing extra core material when a core material is packed (FIG. 5(b)) is necessary and the formation cost is increased.
Moreover, to constitute a three-dimensional circuit, it is necessary to remove extra material for each layer.
Furthermore, even if a step of removing extra material added, a core having a predetermined dimension cannot be obtained when the accuracy of the step is not sufficient. However, it is difficult to control the accuracy.
Moreover, when the extra material is not completely removed, it remains as a waveguide layer. Particularly, when polishing, fine scratches are produced at a waveguide portion, and the scratches cause wave-guided light to scatter. Therefore, such an optical waveguide is not suitable for mass production and it is difficult to reduce the cost of the waveguide.
That is, a conventional optical waveguide has a problem that the waveguide is not suitable for mass production and it is difficult to reduce the cost of the waveguide.